Next: Finding Roots, Up: Polynomial Manipulations [Contents][Index]

### 28.1 Evaluating Polynomials

The value of a polynomial represented by the vector c can be evaluated at the point x very easily, as the following example shows:

    N = length (c) - 1;
    val = dot (x.^(N:-1:0), c);

While the above example shows how easy it is to compute the value of a polynomial, it isn’t the most stable algorithm. With larger polynomials you should use more elegant algorithms, such as Horner’s Method, which is exactly what the Octave function `polyval` does.

In the case where x is a square matrix, the polynomial given by c is still well-defined. As when x is a scalar the obvious implementation is easily expressed in Octave, but also in this case more elegant algorithms perform better. The `polyvalm` function provides such an algorithm.

: y = polyval (p, x) ¶
: y = polyval (p, x, [], mu) ¶
: [y, dy] = polyval (p, x, s) ¶
: [y, dy] = polyval (p, x, s, mu) ¶

  Evaluate the polynomial p at the specified values of x.

  If x is a vector or matrix, the polynomial is evaluated for each of the elements of x.

  When mu is present, evaluate the polynomial for (x - mu(1)) / mu(2).

  In addition to evaluating the polynomial, the second output represents the prediction interval, y +/- dy, which contains at least 50% of the future predictions. To calculate the prediction interval, the structured variable s, originating from `polyfit`, must be supplied.

  See also: polyvalm, polyaffine, polyfit, roots, poly.

: polyvalm (c, x) ¶

  Evaluate a polynomial in the matrix sense.

  `polyvalm (c, x)` will evaluate the polynomial in the matrix sense, i.e., matrix multiplication is used instead of element by element multiplication as used in `polyval`.

  The argument x must be a square matrix.

  See also: polyval, roots, poly.

Next: Finding Roots, Up: Polynomial Manipulations [Contents][Index]